drm: Clean-up VBLANK-related callbacks in struct drm_driver

All non-legacy users of VBLANK functions in struct drm_driver have been
converted to use the respective interfaces in struct drm_crtc_funcs. The
remaining users of VBLANK callbacks in struct drm_driver are legacy drivers
with userspace modesetting.

All users of struct drm_driver.get_scanout_position() have been
converted to the respective CRTC helper function. Remove the callback
from struct drm_driver.

There are no users left of get_vblank_timestamp(), so the callback is
being removed. The other VBLANK callbacks are being moved to the legacy
section at the end of struct drm_driver.

Also removed is drm_calc_vbltimestamp_from_scanoutpos(). Callers of this
function have been converted to use the CRTC instead.

v4:
	* more readable code for setting high_prec (Ville, Jani)
v2:
	* merge with removal of struct drm_driver.get_scanout_position()
	* remove drm_calc_vbltimestamp_from_scanoutpos()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Yannick Fertré <yannick.fertre@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123135943.24140-22-tzimmermann@suse.de